1. Field of the Invention
This invention relates to heavy duty pneumatic radial tires, and more particularly to a heavy duty pneumatic radial tire which effectively prevents the occurrence of uneven wear at shoulder ribs in a tread of the tire.
2. Related Art Statement
A the conventional heavy duty pneumatic radial tire, is for example shown in FIG. 6. Such a heavy duty pneumatic radial tire 1 comprises a tread 2 contacting the ground, a belt 3 composed of plural belt layers containing steel cords therein for the reinforcement of the tread 2, and a carcass 6 located inside the belt 3 and extending between a pair of bead cores 5.
In the tread 2 of the heavy duty pneumatic radial tire 1, both end portions 3a of the belt 3 reinforce both shoulder portions 2a of the tread 2, while the central portion 3b of the belt 3 reinforces the central portion 2b of the tread 2. However, the rigidity of the shoulder portion 2a in the tread 2 is smaller than that of the central portion 2b. Furthermore, there is a dragging phenomenon based on the difference in the size between the central portion 2b and the shoulder portion 2a in the tread 2. Therefore, as shown in FIG. 6, the shoulder portion 2a of the tread 2 is prematurely worn to generate a stepwisely uneven wear or a so-called shoulder wear. In some occasions, the outer surface of the shoulder portion 2a in the tread 2 is wavily worn in the circumferential direction (not shown), which propagates toward the central portion of the tread to generate the uneven wear called as a wavy wear.
In order to prevent the occurrence of these uneven wear types there are proposed the increase of the rigidity in the island part 8 of the shoulder portion 2a of the tread 2, the enlargement of crown radius R of the tread 2, the arrangement of many sipes on full surface 8a of the island part 8 of the shoulder portion 2a in the axial direction, and the like.
Among these proposals for the prevention of uneven wear, the increase of the rigidity in the island part 8 of the shoulder portion 2a is attained by increasing the width 7 in widthwise direction of the island part 8 of the shoulder portion 2a. In this case, the effect of preventing the shoulder wear is recognized to a certain extent, but when a lateral force applied to the tread end 2c, wear becomes severer, the worn portion having a width W.sub.0 and a stepwise difference t.sub.0 or a so-called edging wear 9 (see FIG. 6) is produced in the tread end 2c, which undesirably progresses into the shoulder wear or wavy wear, so that the sufficient restraining effect is not obtained.
On the other hand, when the crown radius R of the tread 2 is made large in order to reduce the difference in the radius between the shoulder portion 2a and the central portion 2b in the tread 2, the dragging of the shoulder portion 2a of the tread 2 is prevented during the running to prevent the occurrence of uneven wear. In this case, however, the ground contact pressure in the central portion of the tread lowers to generate uneven wear in the central portion.
Moreover, when many sipes are arranged on the full surface of the island part of the shoulder portion in the tread in the axial direction, if the tire is used under severer use conditions, particularly a heavy load condition, a small block defined between the adjoining sipes is worn before and after the sipe in the rotational direction of the tire to cause a so-called heel and toe wear.